Conventionally, in the field of manufacture of semiconductor devices, there are provided various methods of planarizing or polishing the surface of an Si substrate including Si wafers in high quality. Typically, there is CMP (Chemical Mechanical Polishing). In recent years, as a new technique of processing a hard-to-work material, such as SiC, and a solid oxide film, such as optical glass, CARE (CAtalyst-Referred Etching) has been proposed.
CMP is a technique, in which by a surface chemical action caused by an abrasive (an abrasive grain) or by the action of chemical components contained in a polishing liquid, the effect of mechanical polishing (the removal of a surface) caused by the relative motion between an abrasive and an object to be polished is increased to obtain a smoothly polished surface at high speed. Commonly, an object to be polished is held by a member called a carrier, the object to be polished is pressed against a platen (a lap) covered with a polishing cloth or a polishing pad, and then the carrier and the platen are relatively moved to each other for polishing, with slurry containing various chemical components and hard, and fine abrasive grains being supplied. Chemical components change the surface of the object to be polished to improve processing speed, compared with the case of polishing an object to be polished using only an abrasive. CMP significantly reduces fine flaws remaining on the surface or an affected layer remaining near the surface in the case of polishing the object to be polished using only an abrasive, allowing an ideal smooth surface to be obtained. Here, for the abrasive used in CMP, fine particles of colloidal silica (SiO2), ceric oxide (CeO2), or ceric oxide containing lanthanum are mainly used, depending on the material of a work piece. The polishing speed (the processing speed) for SiC substrates by previously existing CMP ranges from 20 to 60 nm/h. However, previously existing CMP uses hard diamond abrasive grains, and thus, the surface is damaged.
However, because producer countries of cerium, which is a rare earth element, are unevenly distributed, the stable supply of cerium has a geopolitical risk. Actually, in these years, the event, in which violent fluctuations in rare earth element prices raised a critical problem to industrial circles, is fresh in memory. It is inevitable to face the problem of the depletion of rare earth element sources in future. In addition, CMP uses fine particles, such as colloidal silica, causing problems, such as expensive costs in treating a polishing liquid and poor compatibility of fine particles with a clean room.
In Patent Document 1, the present inventors propose a catalyst-assisted chemical processing method, in which a work piece is put into a processing liquid, to which molecules containing halogen exhibiting no solubility to the work piece in an ordinary state are dissolved, a catalyst made of a platinum, gold, or ceramic-based solid catalyst is brought into contact with or extremely close to the processed surface of the work piece, and the work piece is processed by eluting halogen compounds, which are produced by a chemical reaction between halogen radicals produced on the surface of the catalyst and atoms on the surface of the work piece. More specifically, in Patent Document 1, an example is shown, in which for a processing liquid, to which molecules containing halogen are dissolved, a hydrogen fluoride solution or hydrogen chloride solution is used to process Si, SiC, sapphire, and others.
This processing method based on the reference plane of the catalyst is an ultraprecision planarization technique termed as CARE by the present inventors. CARE is a processing technique using no abrasives or no abrasive grains and an ideal processing method causing no scratches or introducing no affected layers on a processed surface by processing. However, CARE uses a processing liquid, to which molecules containing halogen are dissolved, specifically a hydrogen fluoride solution, causing necessities of the airtightness of processing spaces and processing facilities for exhaust gas and waste liquids. Thus, there are problems that costs of handling and apparatus are more increased than in CMP. Originally, CARE is a technique developed to aim for highly efficiently and highly precisely processing of a hard-to-work material, such as SiC, without introducing any affected layers. In CARE (HF-CARE) using a hydrogen fluoride solution, SiC and GaN can be processed at high speed at a practical level. However, elution caused by hydrogen fluoride proceeds on recesses on the surface as well and thus, surface roughness is not excellent so much. Moreover, because hydrogen fluoride solutions are hazardous and difficult to treat, it is difficult to use hydrogen fluoride solutions for general purposes.
Meanwhile, Patent Document 2 discloses a processing method, in which using only ultrapure water except inevitable impurities in trace amounts, an ionic product is increased using an electrochemical reaction on the surface of a solid having an ion exchange function or catalyst function, the solid being disposed in the ultrapure water, a work piece is immersed in the ultrapure water with the concentration of hydroxyl groups or hydroxyl group ions being increased, the hydroxyl group ions are attracted to the surface of the work piece using the work piece as an anode or with the potential of the work piece being maintained at high level, and the surface of the work piece is removed, or an oxide film is formed on the surface of the work piece by a chemical elution reaction or an oxidation reaction caused by the hydroxyl groups or hydroxyl group ions.
The processing method described in Patent Document 2 is basically electrochemical machining, in which a high voltage is applied. The key requirement is the hydroxyl group increasing process that increases the concentration of hydroxyl groups or hydroxyl group ions in water. For this hydroxyl group increasing process, the surface of the solid having the ion exchange function or catalyst function is used. However, there are problems that the solid contacts the work piece, is damaged, and attached to the surface of the work piece, for example. Thus, the surface of the solid having the ion exchange function or catalyst function and the work piece are basically in a non-contact state. In this state, the process proceeds as the hydroxyl groups or the hydroxyl group ions, which are produced on the surface of the solid, are supplied to the surface of the work piece. However, in the processing method, no processing reference plane is present. Thus the processing method is not commercially available because a highly precise surface is not obtained, the processing speed is slow, and for other reasons.
Patent Document 3 discloses a processing method for a solid oxide film. The method is a processing method, in which a solid oxide film having one kind or two or more kinds of elements bonded through oxygen or a multicomponent solid oxide film formed of a plurality of oxide films is a work piece, and the surface of the work piece is planarized or processed in an arbitrary curved surface. In the method, a catalytic substance is used as a processing reference plane. The catalytic substance assists the production of decomposition products by hydrolysis, in which a water molecule is dissociated to cut back-bonding between an oxygen element and the other element forming a solid oxide film, and adsorbed to the elements. In the presence of water, the work piece is brought into contact with or extremely close to the processing reference plane. The potential of the processing reference plane is set in a range including a rest potential and producing no H2 and no O2. The work piece and the processing reference plane are relatively moved to each other to remove decomposition products from the surface of the work piece.
The processing method described in Patent Document 3 is CARE (Water-CARE) using only water, which is an epoch-making method. CARE (Water-CARE) uses no abrasives or no abrasive grains, including rare earth elements, at all. CARE (Water-CARE) uses no solutions, which are difficult to be handled, such as hydrogen fluoride, and have a large environmental burden, at all. CARE (Water-CARE) can process a solid oxide film, such as an optical material, without introducing any affected layers.